1. Field of the Invention
The present invention relates to a power supply device for electric discharge machining by an electric discharge machine.
2. Description of Related Art
In a recent electric discharge machine, a voltage is applied between an electrode and a workpiece from a subsidiary power supply circuit to urge generation of an electric discharge between the electrode and the workpiece, and when generation of the electric discharge is detected a machining pulse current of large amount is supplied from a main power supply circuit, as disclosed in JP 46-24678 B. According to this method, it takes a considerable time from the time of application of the voltage between the electrode and the workpiece from the subsidiary power supply circuit to a stable flow of machining current of large amount from the main power supply circuit in response to the detection of the electric discharge to attain a stable electric discharge. Particularly, there is a time lag of several hundreds of nanoseconds in a period from the generation of the electric discharge to the actual flow of the machining current (a rise of the voltage from the main power source) because of operating time of a detection circuit for detecting a generation of the electric discharge and switching elements and a rise time of the machining current from the main power supply circuit. In this period, to maintain the electric discharge, it is necessary to flow the current from the subsidiary power supply circuit. However, if the current is too small, an inductance and a capacitance between the power cable and the gap between the workpiece and the electrode are resonated to vibrate the current to be intermitted.
FIGS. 3a and 3b shows a phenomenon of intermission of the electric discharge. In FIG. 3a, an axis of abscissa represents time and an axis of ordinate represents a voltage between the electrode and a workpiece as the other electrode, and VS denotes a voltage applied from a subsidiary power supply circuit and VM denotes a voltage applied from the main power supply circuit for machining. In FIG. 3b, an axis of abscissa represents time and an axis of ordinate represents a current between the electrodes.
At time t1, the voltage VS is applied between the electrodes from the subsidiary power supply circuit and an electric discharge occurs between the electrodes at time t2. A slight leakage current flows between the electrodes from the time t1 to the time t2, and when the electric discharge occurs at the time t2 the current between the electrodes increases and the voltage between the electrodes decreases. The increase of the current or the decrease of the voltage between the electrodes is detected by an electric discharge generation detection circuit and the machining voltage VM is applied between the electrodes from the main power supply circuit. An appearance of the voltage VM between the electrodes is delayed by several hundreds of nanoseconds by operating time of the electric discharge generation detection circuit and the switching elements and rise time of the machining current from the main power supply circuit due to an inductance component of the power cables, etc. Due to the delay, the main voltage VM is actually applied between the electrodes at time t4, as shown in FIG. 3a. The electric current vibrates by the inductance and the floating capacitance of the power cable and the gap between the electrodes to cause a phenomenon of intermitting the electric discharge, i.e. the electric discharge current drops to “0” at time t3 by the vibration of the current in a delay period from the time t2 to the time t4, as shown in FIGS. 3a and 3b. Since the electric discharge is intermitted, the electric discharge for machining can not be generated by application of the main voltage VM from the main power supply circuit.
It may be considered to maintain the electric discharge for preventing the intermission of the electric discharge by increasing the current from the subsidiary power supply circuit. FIGS. 4a and 4b show a state where the current supplied from the subsidiary power supply circuit is increased. In FIG. 4a, an axis of abscissa represents time and an axis of ordinate represents a voltage between the electrodes. In FIG. 4b, an axis of abscissa represents time and an axis of ordinate represents a current between the electrodes. In this case, the electric discharge is maintained not to be intermitted in the delay period from the time t2 at which the electric discharge is generated to the time t4 at which the main current from the main power supply circuit rises, so that the machining current I is flown from the main power supply circuit. However, since the leakage current between the electrodes when applying the voltage from the subsidiary power supply circuit is increased from the time t1 to the time t2 especially in the case where water is used as machining fluid, so that electric corrosion of the workpiece and adhesion of material of the electrode on a surface of the workpiece are increased to deteriorate quality of a machined surface.
Further, there is known an arrangement for preventing the intermission of electric discharge by connecting a series circuit composed of a capacitor and an inductor between the electrode and the workpiece in parallel so that the capacitor is charged when a voltage from the subsidiary power supply circuit is applied between the electrodes and when the electric discharge is generated a current from the capacitor is flown between the electrodes, to prevent the intermission of the electric discharge even if the electric discharge vibrates in the delay period from the generation of the electric discharge to the time of rise of the main current from the main power supply circuit, from JP 59-161230 A for example. In this arrangement, there rises a problem of delay of rise of the voltage from the subsidiary power supply to lower frequency of electric discharge. Further, in a case where an electric discharge is generated immediately after the application of the voltage from the subsidiary power supply, the capacitor is not sufficiently charged with electricity to fail in effectively preventing the intermission of the electric discharge.